A box contains $5$ red marbles, $5$ green marbles, and $11$ blue marbles. If a marble is randomly chosen, what is the probability that it is not red?
Solution: There are $5 + 5 + 11 = 21$ marbles in the box. There are $5$ red marbles. That means $21 - 5 = 16$ are not red. The probability is $ \frac{16}{21}$.